Recently, thin-film photoelectric conversion devices formed from gases by a plasma CVD method have been of interest. Examples of such thin-film photoelectric conversion devices include silicon-based thin-film photoelectric conversion devices made up of silicon-based thin films, thin-film photoelectric conversion devices made of a CIS compound (CuInSe2) and a CIGS compound (Cu(In,Ga)Se2), and the like. Development of the photoelectric conversion devices is being promoted and the production thereof is being increased. These photoelectric conversion devices have the following characteristic. On a large-area and low-cost substrate, semiconductor layers or metal electrode films are stacked by a film deposition apparatus such as plasma CVD apparatus or sputtering apparatus, and thereafter photoelectric conversion devices fabricated on the same substrate are separated and connected by laser patterning to thereby hold a possibility of enabling both lower cost and higher performance of the photoelectric conversion devices to be achieved.
One of the structures of such thin-film photoelectric conversion devices is a stack-type photoelectric conversion device structure that can effectively utilize incident light. The stack-type photoelectric conversion device structure refers to a structure in which a plurality of photoelectric conversion layers receive respective portions of an incident light spectrum. These photoelectric conversion layers use semiconductor materials having respective forbidden band widths suitable for absorbing respective wavelength bands of the incident light, and are stacked, from the light incident side, in the order that a layer of a larger forbidden band width is followed by a layer of a smaller forbidden band width. In this stack-type photoelectric conversion device structure, light of a shorter wavelength is absorbed by a photoelectric conversion layer of a larger forbidden band width, and light of a longer wavelength is absorbed by a photoelectric conversion layer of a smaller forbidden band width. Thus, as compared with a photoelectric conversion device having only one photoelectric conversion layer, the stack-type photoelectric conversion device enables solar light of a wider wavelength range to contribute to photoelectric conversion, and thereby enable the photoelectric conversion efficiency to be improved.
Japanese Patent Laying-Open No. 11-243218 (hereinafter referred to as “PTL 1”) discloses a stack-type photoelectric conversion device in which an amorphous silicon is used as an i-type layer of a first pin junction on the light incident side, a microcrystalline silicon is used as an i-type layer of a second pin junction, and a microcrystalline silicon is used as an i-type layer of a third pin junction. Accordingly, light can be effectively used to achieve a high photoelectric conversion efficiency and the influence of optical degradation of the i-type amorphous silicon can be reduced to improve the photoelectric conversion efficiency after optical degradation.
In addition, as a triple-junction stack-type photoelectric conversion device, a stack-type photoelectric conversion device (a-SiC/a-SiGe/a-SiGe) is known (for example, Japanese Patent Laying-Open No. 10-125944 (hereinafter referred to as “PLT 2”)) in which amorphous silicon carbon is used as an i-type layer of a first pin junction on the light incident side, amorphous silicon germanium is used as an i-type layer of a second pin junction, and amorphous silicon germanium of a smaller forbidden band width than the i-type layer of the second pin junction is used as an i-type layer of a third pin junction.
For these photoelectric conversion devices, it is necessary to form semiconductor materials having respective forbidden band widths suitable for absorbing respective wavelength bands, and it is particularly necessary to manufacture a thin film of a semiconductor material with a high quality that has a narrow bandgap (forbidden band width) such as amorphous silicon germanium. Here, as a method for forming such a semiconductor material of a narrow bandgap, there is a method that varies the flow ratio between a material gas of silicon and a material gas of germanium. Further, Japanese Patent Laying-Open No. 2002-270875 (hereinafter referred to as “PTL 3”) for example discloses a method for controlling the bandgap in the thickness direction of a semiconductor thin-film by varying the frequency of high-frequency electric power applied for generating a plasma for a plasma CVD method.